Introductory Topics

PSY 211

8-29-07

Scientific Method

Why Statistics?

• Common sense is often mistaken

• Cognitive biases:– Hindsight bias*– Overconfidence bias*– Positive illusions*– Confirmation bias– Fundamental attribution error– Errors in emotional prediction– Me-too fallacy

Science

• Common sense and past findings can serve as theory for new studies– Working model designed to organize or

predict behaviors and events– A narrative (story) explaining why things are

what they are

• Test theories using scientific method

Scientific Method*

1. Theory

2. Hypotheses made

3. Design study

4. Run study

5. Analyze results

6. Draw conclusions

7. (Make new, better theories)

Types of Studies

Correlational Studies

• Examines whether two variables are associated (related)

• Correlation coefficient (r): -1 to +1

• Sign indicates direction of relationship

• Magnitude indicates strength of relationship

• Examples: Surveys, ratings, performance, physical measurements

• Is depression correlated with anxiety?

PSY 100… Correlation ≠ causation

PSY 211… Correlation = causation sometimes

Experimental Studies

• Rather than observing two variables, we manipulate one and observe the other

• Independent variable: experimentally manipulated– Control condition vs. Experimental condition– Participants randomly assigned to condition

• Dependent variables: outcome variable, depends on the independent variable

• Examples: drug trials, interventions

Does Prozac decrease depressive symptoms?

IV: Treatment Control

DV:

Of course, experiments have flaws too…

Reporting Results

Statistics

• Definition: using numbers to summarize or interpret information

• Population: large group of interest– “Adults” “Americans” “Depressed children”

“Europeans over 65 at risk for heart disease”– Studies are designed to answer questions

about a population– Problem: unless the population is very small,

not all will be able to participate in the study

• Sample: small group similar to the population; participates in the study– Stratified sample– Random sample– Convenience sample

• Use numbers to describe groups– For populations, numbers called “parameters”– For samples, called “statistics”

PARAMETERS STATISTICS

= M

= SD

Sampling Error

• Samples usually do not perfectly represent the population

• Sample statistics usually differ from the population parameters– Sampling error– “chance” “fluctuations” “bad draw”

• Sometimes sampling error is big; sometimes small… it’s all probability

Reporting Results

• Descriptive statistics: used to describe the sample– M, median, mode, SD, r – Early focus in this class

• Inferential statistics: used to draw conclusions about the population– Significance tests, confidence intervals– Late focus in this class

Homework #1

• Continuous variables:– Ordinal scale: rankings (1st, 2nd, 3rd, etc.)– Interval scale: ranked, and space between

scores are the same size– Ratio, ranked, space between scores are the

same size, have absolute zero point

• Categorical variables:– Nominal scale: labeled groups

• Follow the guidelines for easy points

• Brainstorm ways your questions could be misinterpreted or leave people out

• Ask a friend/classmate to try to find flaws in your questions, and throw out bad ones

• Use available references

• If stumped, brainstorm by checking this site (no copying):http://ipip.ori.org/newIndexofScaleLabels.htm

There have been times in life when I wished everything was over:

Suicidality is associated with a number of important variables, such as stress, depression, hopelessness, and maladjustment (Myers, 2001). This item measures suicidality because suicidal people want to give up on life. Because people may be defensive in responding, I tried to soften the item by avoiding the word “suicide”.

1 2 3 4 5Completely

False

Mostly

False

Neutral Mostly

True

Completely True